Botulinum toxins (also known as botulin toxins or botulinum neurotoxins) are neurotoxins produced by Clostridium botulinum bacteria. Botulinum toxins produce paralysis of muscles by preventing synaptic transmission or release of acetylcholine across the neuromuscular junction. The action of botulinum toxins essentially blocks signals that normally would cause muscle spasms or contractions, resulting in paralysis.
There are eight naturally occurring serologically related botulinum toxins, seven of which are known to cause paralysis (viz., botulinum neurotoxin serotypes A, B, C, D, E, F and G). Each of these serotypes is distinguished by neutralization with type-specific antibodies. However, the molecular weight of the botulinum toxin protein molecule is about 150 kD for all seven of these active botulinum toxins. As released by the Clostridium botulinum bacteria, the botulinum toxin is present in a complex comprising the 150 kD botulinum toxin protein molecule along with associated non-toxin proteins. The total size of the complex may vary. For instance, the botulinum toxin type A complex can be produced by Clostridium botulinum bacteria as 900 kD, 500 kD and 300 kD complexes. Botulinum toxin types B and C complexes are only produced as 700 kD or 500 kD complexes. Botulinum toxin type D complexes are produced as both 300 kD and 500 kD complexes. Botulinum toxin types E and F complexes are only produced as 300 kD complexes. The complexes are believed to contain non-toxin hemagglutinin protein and non-toxin and non-toxic non-hemagglutinin protein. These two non-toxin proteins (which along with the botulinum toxin molecule comprise the relevant neurotoxin complex) are believed to provide stability against denaturation to the botulinum toxin molecule and protection against digestive acids when toxin is ingested.
While botulinum toxin is the most lethal naturally occurring toxin known to man, it has found extensive use as both a therapeutic and a cosmetic agent. For example, in 1986, the feasibility of using type A botulinum toxin for treatment of movement-associated wrinkles in the glabella area was first demonstrated by Schantz and Scott, in Lewis G E (Ed) Biomedical Aspects of Botulinum, N.Y.: Academic Press, 143-150 (1981). The use of botulinum type A for the treatment of wrinkles was published in 1992 (Schantz and Scott, in Lewis G. E. (Ed) Biomedical Aspects of Botulinum, N.Y.: Academic Press, 143-150 (1981)), and by 1994, other movement-associated wrinkles on the face were being treated with type A botulinum toxin (Scott, Ophthalmol, 87:1044-1049 (1980)). The demand for cosmetic botulinum toxin treatments has grown steadily over the years, with current annual sales of botulinum toxin in the United States exceeding $1 billion dollars per year.
One challenging aspect of manufacturing commercial botulinum toxin formulations is stabilizing the botulinum toxin. Like many proteins, botulinum toxin may be degraded or denatured by environmental factors, such as heat, alkaline conditions, mechanical shear forces, or contact with reactive surfaces or substances. Furthermore, the difficulty in stabilizing the botulinum toxin in commercial formulations is exacerbated by the extreme toxicity of the toxin, which permits only minute amounts of toxin to be used for therapeutic purposes. If the botulinum toxin formulation is not properly stabilized, the minute amounts of botulinum toxin may undergo unwanted reactions and/or adhere to the inner surfaces of its storage containers, leading to unacceptable loss of botulinum toxin or activity.
Commercial botulinum toxin formulations are often distributed as lyophilized (i.e. freeze dried) or vacuum-dried powder, in order to prevent degradation and make the botulinum toxin formulation easier to handle and less expensive to transport. Prior to use, botulinum toxin powder formulations are reconstituted with a liquid carrier, such as water or a saline solution. For instance, one commercially available botulinum toxin formulation is sold under the trademark BOTOX® (Allergan, Inc., Irvine, Calif.). The BOTOX® formulation is distributed as a vacuum-dried powder stored in individual vials, each of which contains about 100 units (U) of Clostridium botulinum toxin type A complex, 0.5 milligrams of human serum albumin and 0.9 milligrams of sodium chloride. It has been reported that commercial botulinum toxin formulation must be stored at a temperature of −10° C. or less to maintain the labeled potency for the one year shelf life.
In commercial formulations of botulinum toxin, human serum albumin is often added as a bulk carrier and stabilizer. Generally, albumin may stabilize a therapeutic protein (e.g., botulinum toxin) by one or more of the following mechanisms: (1) reducing adhesion of the therapeutic protein to the inner surfaces of storage or dispensing containers, which include glassware, storage vials, and the syringe used to inject the pharmaceutical composition; and (2) reducing denaturation of the therapeutic protein, especially after reconstituting to prepare a solution of the therapeutic protein. Human serum albumin has the added advantage of being minimally immunogenic, which lessens the likelihood that a human patient will develop antibodies against the botulinum toxin formulation.
Although human serum albumin has been adopted as a stabilizer in commercial botulinum toxin formulations, there are still significant problems associated with this approach. One serious problem is that albumin is derived from blood and is therefore susceptible to carrying blood borne pathogens or infectious agents. For instance, the human serum albumin may carry the Human Immunodeficiency Virus (HIV). Albumin may also carry prions, which are proteinaceous infectious agents that are responsible for causing a neurodegenerative disorder known as Creutzfeldt-Jacob disease. The prions cause misfolding of proteins in the brain, resulting in dementia, memory loss, speech impairment, loss of motor coordination, and death, often within the span of months after the initial onset of symptoms.
Attempts to replace human serum albumin with non-proteinaceous stabilizers generally have been met with difficulties. A non-proteinaceous polymer may be reactive towards the botulinum toxin or may contain reactive impurities that degrade and/or denature the botulinum toxin. For example, some studies have used a poloxamer as a non-proteinaceous stabilizing compound for botulinum toxin. However, these studies report that reconstituted poloxamer-stabilized botulinum toxin formulations demonstrate low botulinum toxin activity, suggesting that the poloxamer excipient either failed to properly stabilize the botulinum toxin and/or induced unwanted degradation reactions to occur.
Accordingly, it would be highly desirable to have a botulinum toxin formulation that is stabilized, but without a proteinaceous excipient, especially without any animal-protein based excipients. Furthermore, it would be highly desirable to have a non-proteinaceous stabilizing excipient that does not itself react with botulinum toxin.